The loss factor—also known as loss angle or tan delta—is a measure of the deviation of the electric behaviour of a real capacitor with respect to that of a perfect system.
In effect, the current flowing through a perfect capacitor with an alternating voltage applied to it (which can be represented in a plane by a vector) is in quadrature relative to the voltage, that is to say, it corresponds to a vector displaced at 90 degrees from the voltage vector.
In a real capacitor, on the other hand, comprising a dielectric material which is not perfect (and whose conductance is therefore not zero), the voltage and current are displaced at an angle of less than 90 degrees, and the difference between 90 degrees and the real displacement angle is an angle denoted by delta.
The loss factor (or tan delta) is the tangent of the angle delta.
As regards the techniques for measuring tan delta, in particular on electric cables, there are two known solutions.
A first solution involves detecting the leakage current, representing the conduction current flowing through the cable insulation, so as to estimate its displacement relative to the voltage applied to the cable (in practice, that means measuring impedance using bridge methods).
This technique has the disadvantage of being relatively unreliable and imprecise, since the leakage current normally has a very low intensity, thus posing a problem of sensitivity for the sensors used to detect it. Another disadvantage is that this technique cannot be used on apparatus which is in service, that is, live, but only on apparatus which is out of service (offline measurements).
A second solution involves detecting an electrical field in order to derive the conduction current that flows through the cable insulation.
This solution has two drawbacks.
First of all, it is not very reliable since the field measurement is influenced by noise outside the cable. Further, this technique is not applicable online, that is to say, when the cable is live.
It is also known patent document US2005/0212524, which describes an electric power line on-line diagnostic method for determining the present condition of a line.
The method comprises the step of measuring the phase angle difference between the load currents at the two ends of the line, i.e. at a first end and at a second end, respectively.
The phase angle difference is obtained by measuring directly the current at the first end and at the second end.
According to the teachings of US2005/0212524, the phase angle difference is related to the loss factor (or tan delta) according to an empiric relation defined by a graph.
However, the phase shift of the load current flowing in the cable, between the phase of the current at the two ends of the cable, is only indirectly and partially related to the value of loss factor (tan delta) in the cable.
Therefore, the teachings of US2005/0212524 do not allow to calculate accurately the value of loss factor (tan delta) in the cable, but, at most, provide a vague indication about said value.
Furthermore, from patent document WO2007/068221 it is known a device for measuring a current value flowing in an electric cable.
Said device comprises two magnetic sensors, one magnetically weakly coupled and the other strongly magnetically coupled with the cable, for measuring the magnetic field in a same section of the cable.
However, WO2007/068221 does not provide any teaching for measuring the loss factor.
Furthermore, from patent document EP 1 892 534 it is known a diagnostic system to facilitate identification and location of insulation defects along a power cable.
Said method comprises the step of: i) connecting an alternating voltage source and applying a voltage source to a cable at a sending end thereof, ii) applying a voltage to the cable at a first frequency to set up a travelling wave along the cable that is reflected at the receiving end thereof, iii) measuring the total complex power loss at the sending end of the cable, iv) measuring the standing wave voltage at any points/section of the cable based on the load impedance connected at the receiving ends of the cable, and on the characteristic impedance of the cable.
According to the method, in order to measure the dissipation factor (tan delta), the method comprises the step of varying many parameters (e.g. the load impedance, the frequency of the voltage source, etc.).
However, the above described method is quite complicated because it needs to connect a plurality of devices/apparatus (e.g. the variable frequency generator) to the ends of the power cable.
This invention has for an aim to provide an instrument and a method for measuring the loss factor of an electrical apparatus having an axially extending elongate geometry and which overcome the above mentioned disadvantages of the prior art.
More specifically, this invention has for an aim to provide an instrument and a method for measuring in a highly reliable and precise manner the loss factor of an electrical apparatus having an axially extending elongate geometry and which can also be used on apparatus which is in service.
A further aim of this invention is to provide an instrument and a method for measuring the loss factor of an electrical apparatus having an axially extending elongate geometry and applicable also to shielded electric cables (without having to remove the shield).